1. Field of the Invention
The invention pertains to the use of white oil having a select viscosity as an extrusion lubricant for vinyl polymers, especially polyvinyl chloride (PVC). In practice, the invention is directed to an extrusion process that has improved external lubrication thus lowering power requirements and processing costs; and to a PVC composition having improved tensile and modulus strength, important in certain applications such as, for example, PVC potable water piping, to which end use the invention is especially suited.
2. Description of the Prior Art
The commercial importance of polyvinyl chloride is due in large measure to its adaptability to a broad spectrum of different use settings. It can be heavily plasticized and thereby made soft and flexible; or it can be left unplasticized for applications requiring a rigid material. Rigid PVC in this regard is widely employed as a material of construction for siding, window profiles, fences, doors and, importantly, pipes, including pressurized pipes. In addition to having adequate rigidity to resist bursting, and conversely to permit such pipes to be buried without fear of their being crushed by overburden, the PVC base resin itself is also sufficiently inert to allow such pipes to carry potable water.
One of the difficulties attendant rigid PVC piping, however, is in the making of same: PVC base resin is recognized to be brittle, hard and very difficult to process. For pipe fabrication, extrusion is conventionally used. But the high temperatures typically needed to extrude PVC can cause premature fusing of the material; and operation at lower temperatures are uneconomical inasmuch as the higher shear rates required translate into commensurately higher power costs.
Remediation of this problem has generally focused on the addition of lubricants, often compounded into the PVC resin, which ameliorate rheolological behavior. The use of lubricants, however, must be done with deliberation: they can not adversely affect the mechanical integrity of the pipe, nor alter the final chemical quality of same thus rendering it unuseable for certain applications. For example, certain lubricants leach, which, if toxic or malordorous, would render them unsuitable for potable water piping. Indeed, products used in drinking water installations, including rigid PVC piping, is subject to various criteria promulgated by the National Sanitation Foundation.
Conventionally, lubricants are divided into two separate classes: internal lubricants and external lubricants. Internal lubricants, typically comprised of metal stearates such as calcium stearate, operate by improving inter-particulate flow when PVC is in molten form. They typically have high compatibility with PVC and are often polar.
Much more directly concerned with the extrusion process, however, are the external lubricants: these are used to reduce the tendency of the PVC to stick to the metal surfaces of the processing equipment, and to delay the fusion of the PVC powder into a melt. External lubricants typically have low compatibility with PVC and are usually non-polar. Unlike internal lubricants, external lubricants eventually migrate to the melt surface of the PVC hence providing lubrication between the polymer and the metal surfaces of the extrusion equipment.
External lubricants are prevailingly waxes, conventionally falling within three classes: paraffin waxes, intermediate waxes and microcrystalline waxes. While useful, these nonetheless suffer from the fact that they are solids in the ordinary course, having high melting points, in some cases above 160xc2x0 C. An example of a wax having industrial popularity as an external lubricant is hard paraffin wax, which has a melting point of 65-76xc2x0 C. (ASTM D-87) and a viscosity (Saybolt Absolute Seconds, SAS) of 48-52 at 210xc2x0 F. (ASTM D-1145). Because they are solid, the utilization of waxes invariably requires them to be either melted or powdered/granulated prior to admixing with the PVC resin. This increases processing costs by requiring further equipment and consumption of additional energy.
Liquid external lubricants, such as mineral oil and silicone oil, are known. However, these are less favored insofar as they have proven less effective in retarding fusion than the waxes. Mixtures of waxes with liquid external lubricants are also known, see e.g. U.S. Pat. No. 3,666,700 which employs a lubricant system of white oil and a microcrystalline wax. White oils are pharmaceutical grade mineral oils that are particularly well adapted to potable water installations insofar as they meet the requirements of USP 23 and relevant FDA regulations provided in e.g. 37 CFR 172.878 and 178.3620(a). In the practice disclosed by U.S. Pat. No. 3,666,700, the white oil has a maximum viscosity of about 500 S.U.S (Saybolt Universal Seconds) at 100xc2x0 F. (about 95.4 centistokes, cSt, at 40xc2x0 C.), with far lower viscosities being specifically preferred, i.e. viscosities in the range of between 50 and about 100 S.U.S. at 100xc2x0 F. Indeed, the joint wax-white oil practice described in U.S. Pat. No. 3,666,700 expressly discourages against using white oils having a viscosity beyond the maximum aforesaid.
Notwithstanding these efforts, the use of waxes, even with the drawbacks hereinbefore described, is still industrially predominant, indicating an on-going need to develop external lubricants for rigid PVC pipe extrusion that offer improved processing, preferably with a reduction in costs.
In satisfaction of the foregoing desiderata, the present invention is directed to a polyvinyl chloride composition containing a high viscosity white oil, namely one having a viscosity of at least about 100 cSt at 40xc2x0 C. The invention is further directed to a process for extruding said composition, and to the articles thus made, including siding, window profiles, fences, doors and, in a particular embodiment, pipe, especially pressurized pipe for potable water.